Phosphoric acid manufacturing with converted waters

Abstract

Generation, use, and treatment of new and unique process waters in or for wet-process phosphoric acid manufacturing to substantially eliminate discharge of contaminants to the environment. Acid pond water from conventional wet-process phosphoric acid manufacturing, contaminated with compositions containing fluorine, heavy metals, and radioactive elements such as radium-226, plus residual P 2 O 5 , is superseded. Substantially all contaminants from the wet-process are insolubilized and removed, whereas P 2 O 5 normally lost is recovered. Decontaminated gypsum stack water with a mildly acidic pH results from stepwise raising of wastewater pH, with clarification, and subsequent re-acidification, to provide a decontaminated process water with slightly acidic pH adapted to match the pH of environmental ground water. The process is made more economical, while enhancing protection of surrounding air, soil, and water from contaminants.

We claim: 
     
       1. In wet-process phosphoric acid manufacturing productive of waste gypsum and water containing soluble fluorides, metal salts, and radioactive compositions as contaminants, wherein said waste gypsum was slurried and stacked for drainage into a pond of water, said drainage water having a pH of at most about 2 and containing said dissolved contaminants as well as residual phosphoric acid,   the improvement comprising superseding the aforesaid acid gypsum pond drainage by slurrying waste gypsum with, and thereby limiting gypsum stack drainage to, water substantially freed of said dissolved contaminants and having a more nearly neutral pH of about 6 to 7.   
     
     
       2. The manufacturing improvement according to claim 1, wherein before gypsum slurrying and stacking, wet-process gypsum filter cake is washed last with water partially neutralized from a starting pH of at most about 2 and clarified with removal of precipitated contaminants in the underflow, then alkalized to a pH of at most about 12 and clarified with removal of precipitated contaminants in the underflow, and then re-acidified to a nearly neutral acidic pH. 
     
     
       3. The manufacturing improvement according to claim 2, wherein the pH of the partially neutralized water is about 4.5, and the pH of the alkalized water is about 11. 
     
     
       4. The manufacturing improvement according to claim 3, wherein re-acidified water is used also as make-up for other water usage in the wet-process manufacturing. 
     
     
       5. The manufacturing improvement according to claim 4, wherein the re-acidified water not otherwise used in the wet-process is discharged into ground water having a matching pH. 
     
     
       6. The manufacturing improvement according to claim 2, wherein the underflow from the partial neutralization and clarification is segregated as contaminated sludge. 
     
     
       7. The manufacturing improvement according to claim 6, wherein the underflow from the alkalization is recycled into water with a pH of at most about 2 before partial neutralization and clarification. 
     
     
       8. Modified wet-process manufacturing of phosphoric acid, wherein phosphate rock is reacted with sulfuric acid in a reactor and wherein by-product gypsum filter cake is washed repeatedly then is slurried and finally is stacked to drain, comprising forming separate process waters containing contaminants by condensing steam and other gases from wet-process flash coolers and evaporators, and by scrubbing wet-process vent gases;   recovering P 2  O 5  from process leaks, spills, and washes with one of the process waters, clarifying it, returning the underflow to the reactor, and preliminarily washing the gypsum filter cake with the clarified water; and   raising the pH of the other process water, in a first stage toward neutral and in a second stage beyond neutral, clarifying it, removing precipitated contaminants in the underflows from both stages, re-acidifying the clarified water to nearly neutral pH about 6 to 7, and washing the wet-process by-product gypsum filter cake with the clarified re-acidified water.   
     
     
       9. Modified wet-process manufacturing of phosphoric acid, according to claim 8, including collecting gypsum stack drainage water and alkalizing it in the second stage before re-acidification. 
     
     
       10. Modified wet-process manufacturing of phosphoric acid, according to claim 8, wherein the underflow from the first stage is segregated as contaminant-containing sludge, and the underflow from the second stage is recycled to the first stage. 
     
     
       11. Modified wet-process manufacturing of phosphoric acid, according to claim 8, wherein over-all wet-process water-soluble P 2  O 5  loss is limited to about one percent. 
     
     
       12. Method of producing by-product gypsum collected as filter cake, slurried, and stacked to drain in wet-process phosphoric acid manufacturing, substantially free of contaminants, comprising treating process water contaminated with dissolved fluorine, heavy metals, and radioactive compositions as well as residual P 2  O 5 ,   in a first stage of partial neutralization and clarification, including removing underflow containing, those of said precipitated contaminants insoluble at the pH to which partially neutralized, and   in a second stage of alkalization and clarification, including removing underflow containing, remaining said precipitated contaminants soluble at the pH to which partially neutralized but insoluble at the pH to which alkalized, and   in a stage of re-acidifying to nearly neutral pH about 6 to 7, producing a slightly acidic, non-scaling, pH-adjusted water (pH-AW).   
     
     
       13. Method according to claim 12, wherein the pH reached in the first stage is about 4.5, precipitating most fluorides and about one-third of the phosphates, and most heavy metals and radium-226. 
     
     
       14. Method according to claim 12, including reducing dissolved fluoride and fluosilicic content of the process water from a starting concentration of about ten thousand ppm to a final concentration of about two ppm. 
     
     
       15. Method according to claim 12, including reducing residual phosphoric acid content of the process water from a starting concentration of about twenty thousand ppm to a final concentration of about twenty ppm. 
     
     
       16. Method according to claim 12, including producing an acid process water from pH-AW while collecting condensed process steam and entrained phosphoric acid, and using the acid process water in preliminarily washing gypsum filter cake to be slurried and stacked. 
     
     
       17. Method according to claim 12, including using pH-AW in final washing of gypsum filter cake to be slurried and stacked, as well as make-up water elsewhere in the wet-process manufacturing. 
     
     
       18. Method according to claim 17, wherein the washed gypsum filter cake is slurried with water drained from the resulting stack. 
     
     
       19. Method according to claim 18, wherein water drained from the resulting gypsum stack and not used to slurry gypsum filter cake is sent to the second stage of alkalization and clarification. 
     
     
       20. Wet-process phosphoric manufacturing process modified by producing a decontaminated water as follows: treating gypsum stack water obtained from filter cake water by partially neutralizing it with limestone to a pH of about 4.5 and clarifying it,   then alkalizing the partially neutralized clarified water with lime to a pH of about 11 and clarifying it, and   acidifying the alkalized clarified water to a pH of about 6 to 7, to yield a non-scaling pH-adjusted water and using the resulting pH-adjusted water in the wet-process in at least two of the following ways:   scrubbing wet-process vent gases,   condensing steam from wet-process flash coolers & evaporators,   washing wet-process by-product gypsum filter cake, as make-up water added where needed in the wet-process.